Pulse radar for determining angles of elevation



R. ALFANDARI ET AL PULSE RADAR FOR DETERMINING ANGLES OF ELEVATION FiledAlig. 29, 1967 mm- WXLR E E L L u lm F t m zk mom 'A" A RADAR 1 I,...\'J "J COMPUTER J /U v T o Sheet INVENTORS ROGER ALFANDARI BERNARDDAVEAU Attorney June 3, 1969 I ALFANDARI ET AL 3,448,450

PULSE RADAR FOR DETERMINING ANGLES OF ELEVATION FiledAug. 29, 1967 Sheet2 of 5 j; 2 j PHASE smmns PHASE SHWTERS A ROGER ALFANDARI BERNARD.DAVEAU GM was ATTORNEY June 3, 1969 R. ALFANDARI ETAL 3,448,450

PULSE RADAR FOR DETERMINING ANGLES OF ELEVATION Filed Aug. 29, 1967Sheet 3 01 5 -u--a-la- I g I J I I PHASE CONTROL ROGER ALFANDARI BERNARDDAVEAU by am 6- MTORN'EY June 3, 1969 R. ALFANDARl T 3, 56

PULSE RADAR FOR DETERMINING ANGLES OF ELEVATION Filed Aug. 29, 1967Sheet of 5 AZMUTHAL INVENTORS:

ROGER ALFANDARI BERNARD DAVE-AU ATTORNEY June 3, 1969 R. ALFANDARI ET AL3,448,450

PULSE RADAR FOR DETERMINING ANGLES OF ELEVATION Filed Aug. 29, 1967Sheet 5 of s INV'ENTORS ROGER ALFANDARI BERNARD DAVEAU by R0 S ATTORNEYUnited States Patent Ofice Int. Cl. ols 9/02 US. Cl. 343-16 12 ClaimsABSTRACT OF THE DISCLOSURE A stack of vertically spaced radar antennas,effective over a narrow azimuthal range, are simultaneously energized,with a plurality of pulse carrier frequencies common to all antennas,through respective phase shifters whose phase shifts are chosen toestablish a desired multilobe pattern, each carrier frequency generatingan individual lobe with its axis inclined at a particular angle ofelevation. These angles are varied, within relatively narrow verticalranges, by concurrent adjustment of the several phase shifters wherebythe multilobe pattern sweeps an extended vertical sector. With athree-lobe pattern, the central lobe may be used for the tracking of atarget while the flanking lobes serve, in cooperation with an associatedazimuthal radar, to keep the system centered on the target.

Our present invention relates to a pulse-type radar system adapted to beused, e.g. in combination with a conventional azimuthal or panoramicradar, to ascertain the angle of elevation of microwave energy reflectedfrom external objects and, thereby, to determine the altitude of thoseobjects. 7

A system of this type is useful for monitoring purposes, to give anobserver a complete three-dimensional picture of the locations ofaircraft or other elevated objects within a particular sector or overthe entire'horizon, as well as for the tracking of a particular target,e.g. when the system is carried aboard a missile.

The general object of our present invention is to provide an improvedradar system of the type referred to which sharply discriminates amongdifferent angles of elevation while using only a limited band of carrierfrequencies.

Another object of our invention is to provide a radar system of thischaracter which, by virtue of its sharp angular directivity, can be usedto supplement the information gained from an associated panoramic radar,particularly in regard to the presence of low-flying craft at greatdistances (up to several hundreds of kilometers) from the observer.

. It is also an object of the instant invention to provide means in suchradar system for minimizing the fading effect normally encountered incertain positions of a target with reference to an array of radarantennas trained thereon.

In accordance with an important feature of our present invention, weprovide an altitude-measuring radar system wherein a stack of verticallyspaced radar antennas, effective over a narrow azimuthal range, aresimultaneously energized with one or more pulsed carried frequencies byway of respective phase shifters so arranged that the microwave energyof a particular carrier frequency is radiated, within a given verticalplane, in a pattern whose main lobe is inclined at a predetermined angleof elevation related to the carrier frequency as well as to the in-Patented June 3, 1969 troduced phase shifts; in the presence of severaldifferent carrier frequencies, a multilobe pattern ensues. Bycontrolledly varying the aforementioned phase shifts, we can angularlydisplace the principal lobe or lobes of the pattern within a sectoralrange whereby a continuous sector, within the selected vertical plane,is rapidly scanned by one or more sharply directive beams. With theantennas acting as both'radiators and receivers, the array has the samesharp directivity for incoming radiation. By a rotatable mounting of thearray, its effective plane can be changed, advantageously in step withthe azimuthal sweep of an associated panoramic radar.

For monitoring purposes, the echo pulses received on each carrierfrequency may be separately evaluated (e.g. fed to a display device) forlocating reflecting objects indifferent sectoral zones of theelevational sweep range; in an alternate arrangement, a three-lobepattern may be used for tracking a specific target in a given azimuthalposition, with the central lobe furnishing the altitude information ofsuch target while the flanking lobes serve to reorient the multilobepattern by so altering the phase shifts as to keep the central lobetrained on the target with changing angles of elevation. The initialorientation of the pattern in each azimuthal position may be determinedby a computer under the control of an associated panoramic radar.

In order to maximize the response of the system to in coming echo pulsesfrom a selected target, we prefer to subdivide the stack of antennasinto two vertically spaced groups from which incoming energy is directedwith additive and subtractive phase relationship into separate outputchannels, such as two conjugate branches of a waveguide junction of themagic-T type. It is also possible to use two parallel waveguides havingjunctions for the additive and subtractive combination, respectively, ofwave energies from the two groups. The waveguide junctions may havedifferent coupling factors or transfer coefficients to provide adesirable distribution of microwave amplitudes over the length of thearray, or of a group thereof, with a maximum energy transmission andreception at the center of the array or group.

The invention will be described in greater detail with reference to theaccompanying drawing in which:

FIG. 1 is a diagrammatic view of a radar system embodying our invention;

FIG. 2 is a perspective view of a branched waveguide adapted to be usedin the system of FIG. 1;

FIG. 3 is a diagrammatic view of two parallel waveguides usable as asubstitute for the branched waveguide of FIG. 2;

FIG. 4 is a view similar to FIG. 1, showing a modified system;

FIG. 5 is a diagram serving to explain the mathematical principlesunderlying our invention;

FIG. 6 is a schematic view of a phase shifter forming part of thesystems of FIGS. 1 and 4;

FIG. 7 illustrates the radiation pattern realizable with a systemaccording to our invention;

FIG. 8 diagrammatically illustrates a preferred pattern of amplitudedistribution in a system according to our invention; and

FIG. 9 is a diagram similar to FIG. 7, showingan alternative sweeppattern.

In FIG. 1 we have shown a stack of a multiplicity of radar antennasdesignated A A .Aj A,, A Each of these antennas, serving for thetransmission and reception of microwave energy, is sharply directive ina horizontal plane and may be in the shape of a conventional dipole,coil, dielectric tube and so forth. The antennas form part of arotatable assembly whose vertical axis of rotation, indicated at 0 inFIGS. 7 and 9, may pass through these antennas or in the vicinitythereof.

The assembly may also include a common reflector, as shown at 14 in FIG.4, in the shape of a parabolic cylinder segment focused on the antennastack. The elements included in the rotatable assembly are shown locatedin FIG. 1 above a horizontal line X -X They further include a waveguide1 having junctions C C C C C respectively coupled with the antennas A Athrough individual phase shifters D D D,- D,, D Each of these couplersmay consist of a pair of oppositely directive junctions to reduce thestanding-wave ratio. The radiated energy may be circularly or linearlypolarized or may be switched between these two types of polarization.

Waveguide 1 is connected through a rotary coupler 3 and a multiplexer 2to a set of microwave oscillators E E 13;; operating on differentwavelengths A 7x, A These oscillation generators are pulsed via a lead50 in synchronism with the pulse source (not shown) of a panoramic radar51 which may operate in a different frequency band and whose antenna orantennas 52 are rotatable in unison with the antennas A A and associatedelements of the altitude-measuring radar. The carrier frequencies fromthese oscillators further serve for the demodulation of received waveenergy, the demodulated echo pulses being fed to a computer 4 which isalso under the control of the panoramic radar 51.

Another rotary coupler 6 connects a sweep-control circuit 5 to a cable53 consisting of a bank of individual conductors respectively connectedto the phase shifters D D for controlling the extent of the phase shiftsintroduced thereby. The output of control circuit 5 is also supplied tothe computer 4. From this information, the computer operates thedeflectors of a cathode-ray tube 54 whose screen shows the distance rand the angle of elevation of a target T whose location, in terms ofdistance r and azimuthal angle a, is also seen on a panoramicoscilloscope screen 55 controlled by the radar 51.

The azimuthal directivity of the antenna array A A should be such as toallow for the scanning of an entire elevational sector in any horizontalposition as determined by the power of resolution of the panoramic radar51.

With suitable adjustment of the phase shifters D D the verticalradiation pattern of antennas A A exhibits as many principal lobes asthere are carrier oscillators E E The axis of each lobe includes acharacteristic angle of elevation, 0 6 with the horizontal, themagnitude of this angle being determined in part by the correspondingcarrier wavelength x and in part by the relative phase shift betweenadjoining antennas of the array. (It will be understood, in thisconnection, that the linear array of antennas A etc. may be replaced bya two-dimensional array, with each aerial shown in FIG. 1 representativeof a plurality of such aerials disposed side by side.)

As illustrated in FIG. 5, two outgoing wave components W W traveling ina direction which has the angle of inclination 0, will be in phase iftheir respective emitters A A operate with a phase difference equal tothe length in radians of the path difference d -sin 0, d being thephysical distance between radiators A and A It may be assumed that thedistance d is constant throughout the array, this condition simplifyingthe calculations while being not otherwise essential.

If the transmitted frequency has a propagation wavelength A along thewaveguide 1, the phase difference Azp between successive junctions C Cis equal to 21rd/A with where A is the free-space Wavelength of theapplied carrier oscillation and w is the cutoff wavelength of thewaveguide. If the phase shifters D D introduce an additional phasedisplacement Ail/ (variable under the control of circuit 5, FIG. 1), thevalue of angle 0 can be determined from the relationship Ago) sln 21rdwhere A p=A1,!/+A\// Thus, a change of either Aip or A will vary theangle 0. The formula also shows, however, that the change of 0 is notproportional to the change in phase shift so that, in practice, thesweep range available by adjustment of the phase shifters D etc. islimited. This limitation is compensated by the fact that severaldiscrete carrier frequencies are simultaneously available to scandifferent sectoral zones Z Z 2;, etc. as illustrated in FIG. 7. Withproper choice of these carrier frequencies, the zone Z swept by the lobeB (corresponding to the shortest wavelength A immediately adjoins thezone Z assigned to lobe B (operating wavelength the latter in turn beingcontiguous to zone Z (wavelength The beam width of these lobes may beregarded as bounded by, say, the -3 db level of its signal strength. Theextreme sweep positions of each beam B B B have been shown at B B B Band B3! B3!!- The phase shifters may be of a conventional, continuouslyadjustable type comprising, for example, ferrite cores or controlledrectifiers. They may, however, also be binary combinations of fixedphase-shifting devices, as illustrated in FIG. 6 where phase shifter D,is shown to consist of four stages with individual phase shifts equal to1r, 1/2, 1r/4 and 1r/ 8, respectively, with possible addition of furtherstages having incremental values equal to 1r divided by higher powers of2. These stages may be selectively actuated, under the control ofcircuit 5, at the beginning of each radar pulse cycle for a progressivephase displacement.

By the use of phase-shifting means rather than variable-frequencygenerators for the purpose of angularly displacing the multilobepattern, particularly with incremental phase shifts through a digitalchain as shown in FIG. 6, the number of sidebands is held to a minimum.

In FIG. 2 we have shown a modified waveguide 10 for the energization ofantennas A A and the retrieval of their incoming microwave energy. Thiswaveguide splits into two branches 11 and 12 respectively coupled to thegroup A of aerials A A; (via phase shifters D D and to the group A" ofaerials A, A (via phase shifters D D,,). The top aerial A; of the lowergroup A and the bottom aerial A of the upper group A" may be spaced fromeach other by the aforementioned distance d, representing the separationof any two adjoining aerials within each group, though the intergroupspacing could also be different. The associated phase shifters D D,,,are however, correlated in the aforedescribed manner to establish asingle preferred direction of radiation, throughout the array A A,,, forany one operating wavelength.

Branches 11 and 12 are joined to each other, and to the common guideportion 10, by a conventional magic-T junction from which a furtherchannel 13 extends in conjugate relationship with channel 10. Incomingradiation of a given frequency, reflected by an external object towardthe two antenna groups, is combined additively in guide channel 10 andsubtractively in guide channel 13 to provide a summative output SO and adifferential output DO. Depending on the relative transit time betweenthe object and the two antenna groups, either the summative output S0 orthe differential output DO may predominate; from a combination of theseoutputs the computer may obtain accurate information on the location ofthe object.

The wazveguide sections 11, 12 are terminated at the top by suitablenonreflective loss material, not shown; such termination is, of course,also provided at the top of waveguide 1 in FIG. 1.

The transfer coefiicients of the couplers C C (FIG. 1), not shown inFIG. 2, may be so chosen that the amplitudes of the outgoing waves (andtherefore also the sensitivity of the system for incoming waves) followa predetermined pattern, with a maximum radiation near the midpoint ofthe array (FIG. 1) or each group (FIG. 2). This has beendiagrammatically illustrated, with particular reference to thearrangement of FIG. 2, in FIG. 8 which also shows the cophasalrelationship of the waves transmitted in a predetermined direction ofpropagation. It may be mentioned in this connection that such parallelrelationship need not always be exactly maintained but that, forexample, the phase shifts introduced between the couplers and theantennas may be slightly modified to provide, instead, a convergence ofthe directions of cophasal propagation upon an object at close range.Conversely, a slight divergence may be desirable in certain instances,e.g. for locating an extended object or group of objects.

The combination of a summative and a differential output may also beobtained with an arrangement as illustrated in FIG. 3 wherein thewaveguide 1 of FIG. 1 is supplemented by a. second waveguide 7 parallelthereto. Junctions C C; of antenna group A are connected to waveguide 7via additional junctions X0 XC respectively, whereas junctions C C ofantenna group A are connected to waveguide 7 by way of additionaljunctions XCj+1 XC respectively, with a phase reversal relative tojunctions XC XC Waveguide 7, therefore, delivers a subtractive ordifferential output DO whereas waveguide 1, as in FIG. 1, supplies anadditive or summative output SO.

Reference will now be made to FIG. 4 for a description of a trackingsystem generally similar to the one shown in FIG. 1 using a splitantenna array A, A as shown in FIG. 2 together with the parabolicreflector 14 previously referred to. Three frequency generators E E E;with respective operating wavelengths A A A are shown connected toWaveguide via multiplexer 2, rotary coupler 3, waveguide 1 and a pair ofduplexers 16, 15 in cascade, duplexer 15 also receiving the additiveoutput SO from waveguide 10 and directing it via an isolatingtransmit-receive unit 18 and a parametric amplifier 20 to ahigh-frequency junction 33 whence this output energy is returned to thefrequency generators E E E, by way of duplexer 16 and elements 3 and 2.The provision of loop 15, 18, 20, 33, 16 appreciably reduces the noiselevel as compared with a simple two-way transmission through a waveguide1, 10 as indicated in FIG. 1. Oscillators E E E pulsed as in theprevious embodiment under the control of an azimuthal radar 51, againserve also as demodulators and supply the echo-pulse signals to a logicnetwork 44 by way of respective gates within a circuit 43 which is underthe control of a selector 42. Elements 42, 43, 44 are part of thecomputer 4 which also includes a coupling circuit 41 for the control ofgate selector 42 in response to information from radar 51, logic network44 designed to keep the central lobe (wavelength trained upon a selectedtarget, and an output network 29 which acts as a monpulse interpolatorin evaluating the target information contained in a combination ofsignals derived from the summative and differential outputs SO and D0 ofantenna array A, A". For this purpose, waveguide channel 13 is connectedby way of an isolating transmit-receive network 17, a parametricamplifier 19 and a filter 21 to a mixer 23, receiving a localoscillation from generator E through a rotary coupler 27 and ahigh-frequency junction 28; the latter junction also feeds a mixer 24receiving incoming energy from waveguide 10 through elements 15, 1 8,20, 3'3 and a filter 22. The LP outputs of mixers 23 and 24 reach thenetwork 29 through respective amplifiers 25, 26 and rotary couplers 32,31. Sweep control circuit 5*, responding to signals from logic network44, supplies altitude information to the network 29.

The operation of the system of FIG. 4 will be explained with referenceto the diagram of FIG. 9 which shows the lobes B B B occupying adjacentpositions in the vertical sweep plane. The group of lobes B B B can bejointly displaced, through adjustment of the phase shifts of elements D1),, (FIG. 2), into different angular positions such as those shown at BB B and B B B Thus, the overlapping sweep zones of FIG. 9 (like thecontiguous sweep zones of FIG. 7) establish, within that vertical plane,a coherent sectoral scan ranging from position B to position B If thecentral lobe B of the radiation pattern is properly trained upon atarget to be tracked, logic network 44 receives an; output fromdemodulator E and keeps the gating circuit 43 operative, via circuits 41and 42, to pass that output. If, within a given azimuthal position ofthe system and at a target distance indicated by radar 51, no echopulses are received on wavelength k network 44 causes the circuit 43 toswitch to demodulators E and F for determining whether the target islocated below or above the central lobe. Sweep-control circuit 5 thenoperates to recenter the lobe B associated with generator E on thetarget by varying the phase shifts of the wave energy supplied toradiators A and A". When the system is thus realigned, output network 29responds to the reappearance of a signal from mixers 23 and/or 24 byinforming the coupling network 41 that gate-control circuit 42 is toreopen the signal path between elements B; and 44. A visual indicator,such as the oscilloscope 54 of FIG. 1, may also be directly controlledby the calculator 29.

Logic circuit 44 stores the information from network 29 to realign thebeam group B B B via control circuit 5, after each rotation of theantenna array with the target observed in the same azimuth-a1 positionduring the preceding sweep, subject to correction of the angle ofelevation in the manner just described to allow for a shift in thetarget position. In this manner, different targets may be individuallytracked in different azimuthal positions. Naturally, an adjustment ofsweep control 5 to train the multilobe pattern upon a particular targetmay also be carried out manually by an operator observing the displayson screens 54, 55 of FIG. 1.

Modifications of the systems described and illustrated, readily apparentto persons skilled in the art, are intended to be embraced within thespirit and scope of our invention as defined in the appended claims.

We claim:

1. A radar system comprising a stack of radar antennas of high azimuthaldirectivity vertically spaced for emitting and receiving microwaveradiation in a common vertical plane; a source of pulsed high-frequencycarrier oscillations of different carrier frequencies; feed means forconnecting said source to said radar antennas, said feed means includingindividual phase shifters in series with said antennas and adjusted toestablish a directive beam pattern in said vertical plane with severalnarrow lobes respectively due to said carrier frequencies, said lobeshaving axes inclined at different angles of elevation; control meansconnected to said phase shifters for concurrently adjusting same to varysaid angle of elevation within respective sweep zones; and evaluationmeans connected to receive incoming wave energy from said antennas byway of said phase shifters.

2. A system as defined in claim 1 wherein said lobes include a centrallobe and a pair of flanking lobes, said evaluation means including anoutput circuit responsive only to echo pulses from said central lobe andcircuit means responsive to echo pulses from said flanking lobes foroperating said control means to keep said central lobe trained upon apulse-reflecting target.

3. A system as defined in claim 2 wherein said feed means compriseswaveguide means with .two output channels for reflected microwave pulsesat the frequency of said central lobe received in additive andsubtractive phase relationship, respectively, from two vertically spacedgroups of said antennas, said output circuit being connected to receiveenergy from both said channels.

4. A system as defined in claim 3 wherein said waveguide means comprisesa waveguide split into two branches merging at a magic-T junction, saidoutput chanphase shifters consists of a plurality of digital stages eachnels being conjugate branches of said junction.

5. A system as defined in claim 3 wherein said waveguide means comprisesa pair of parallel waveguides each provided with an individual set ofjunctions coupling it to each of said antennas by way of the respectivephase shifters.

6. A system as defined in claim 1 wherein said feed means comprises anupstanding waveguide with a plurality of junctions connected to saidantennas by way of the respective phase shifters.

7. A system as defined in claim 6 wherein said junctions aresubstantially equispaced along said waveguide.

8. A system as defined in claim 6 wherein said junctions are couplerswith dilferent transfer coefficients providing a distribution ofmicrowave amplitude progressively increasing along one portion anddecreasing along another portion of said waveguide.

9. A system as defined in claim 1 wherein said stack of radar antennasis provided with a common vertical reflector of parabolicallycylindrical shape.

10. A system as defined in claim 1 wherein each of said phase shiftersconsists of a plurality of digital stages each adapted to introduce aphase shift substantially equal to 1r divided by a power of 2.

11. A system as defined in claim 1, in combination with an azimuthalradar, said antennas being mechanically coupled with said azimuthalradar for rotation about a vertical axis in unison therewith.

12. A system as defined in claim 1 wherein said carrier frequencies andthe effective range of adjustment of said phase shifters by said controlmeans are correlated to establish a coherent sectoral scan by said lobeswithin said vertical plane.

References Cited UNITED STATES PATENTS 3,016,531 1/1962 Tomiyasu et al343l7.l 3,258,774 6/1966 Kinsey 343-400 X 3,274,593 9/1966 Varela et al34316 3,344,426 9/1967 Long.

RODNEY D. BENNETT, JR., Primmy Examiner.

JEFFREY P. MORRIS, Assistant Examiner.

US. Cl. X.R. 343-17.1

